1. Field of the Invention
The present invention relates to methods for measuring the Doppler shift in a detection system, for example a radar or a sonar, using ambiguous codes (Doppler x time). More precisely, it relates to transmission codes for a type of detection system which allow fine measurement of the Doppler shift of the echoes received from a moving target, with a precision adjustable independently of the parameters pertaining to the duration T, the initial frequency f(0) and the final frequency f(T) of the pulse coded according to this system.
2. Discussion of the Background
The modulation of the pulses of a sonar or a radar with particular codes which allow the accuracy of the measurement to be improved is well known and is in particular described at length in the work by A. W. RIHACZEK, entitled "Principles of high resolution radar", published by Mc Graw-Hill, copyright 1969. It is explained therein, in particular, that the transmission codes can be classified in two categories.
Nonambiguous codes, for which the ambiguity function time x Doppler shift has, around the maximum, two main directions parallel to the time and Doppler shift axes. It is then known that, for such codes, with constant (or not significantly modulated) amplitude over the duration of the code, the Doppler tolerance is inversely proportional to the duration of the code, which prevents it from being possible to fix the transmission duration and the Doppler tolerance independently. With the transmission durations used in the sonar technique, such codes lead to a very large number of copies, of the order of 1000.
Ambiguous codes, for which the ambiguity function time x Doppler shift has a peak secant at zero with the time and Doppler axes. This peak is strictly linear in the case of the hyperbolic frequency modulation (HFM) code, which has the feature of being perfectly Doppler-tolerant. In this case, the Doppler tolerance is entirely determined by the bandwidth B of the transmitted code, and is in general insufficient for accurate Doppler measurement.
The linear frequency modulation (LFM) code also belongs to the ambiguous code class. However, once the duration T and the bandwidth B of the code are fixed, the Doppler tolerance is given by, c being the speed of sound: ##EQU1##
This to ace is generally too small compared with needs and entails oversizing the number of reception copies.
Another ambiguous code example is described in U.S. Pat. No. 5,481,504 granted on Jan. 2, 1996 in the name of ATLAS Electronics Gmbh. In this code, the instantaneous frequency is a function of a power of time. This power is given by a parameter which varies between 0 and 1 and, when this parameter is equal at 1, it becomes equivalent to the linear frequency modulation code. In this case, a very large number of reception copies is needed and the minimum number of copies is substantially equal to that of the LFM code.